Modern distribution of audio signals to consumers involves the use of data rate reduction or audio compression techniques to lower the amount of data required to deliver these audio signals to consumers while causing minimal impact to the original audio quality. This reduction in the size of the data translates into a savings of transmission and storage bandwidth, thereby allowing cost savings or carriage of more programs in a given space. Systems including AC-3, DTS, MPEG-2 AAC, and High Efficiency AAC (HE AAC) are examples of common audio data reduction techniques
Auxiliary data including metadata, also known as data about the audio data, is included in these systems to describe the encoded audio. Metadata is multiplexed with the compressed audio data and delivered to consumers where it is extracted and applied to the decoded audio in a sometimes user-adjustable manner to optimize reproduction for individual tastes or listening environments.
Metadata parameters such as dialnorm, program level, dynamic range control (DRC), and others are intended to control loudness and dynamic range, and are generated further upstream in the broadcast process, optimally in the production phase. Metadata has grown in importance as the arbiter of the balance between satisfying proposed loudness mitigation legislation such as the CALM Act and the artistic intent of program producers. Increased metadata reliability would allow satisfaction of existing and proposed legislation while keeping the original content protected and intact for those customers that have the ability and desire to experience it. As most of the signal processing prior to transmission occurs in the non-encoded pulse-code modulation (PCM) domain, carriage and storage of metadata, sometimes in a serial 115.2 kbps RS-485/422 format has conventionally been cumbersome and unreliable.
Professional systems such as Dolby E from Dolby Laboratories and e-squared from Linear Acoustic provide paths for metadata to be transmitted along side multiple channels of audio. However some of these systems suffered from the expense of being a separate process from the audio signal transmission process, and if not used correctly could exacerbate problems with audio/video synchronization (lip sync).
Newer standards such as SMPTE 2020 from the Society of Motion Picture and Television Engineers provide a relatively simple path for metadata to reside inside of the ancillary (VANC) space of serial digital video (SDI) signals, however not every device is capable of passing this VANC data, nor are most systems capable of recording or otherwise storing this data. New headers such as those proposed to work with the broadcast wave format (BWF) can also carry metadata information, however these have not yet been standardized or are not in broad use.